1. Field of the Invention
The present invention relates to a generation device for generating a terahertz wave (electromagnetic wave having a frequency within a range of 0.3 THz to 30 THz in this specification).
2. Description of the Related Art
The terahertz wave (electromagnetic wave having a frequency within a range of 0.3 THz to 30 THz in this specification) has its characteristics mainly as follows.
First, the wavelength is relatively short, and hence like an X-ray the terahertz wave can pass through a nonmetal substance. In addition, there are a lot of biomoleculars and drugs having an absorption spectrum specific to the frequency band of the terahertz wave. Moreover, a pulse width in the time domain is relatively short, and hence the terahertz wave has a spatial resolution that is suitable for a variety of imaging.
As applications of the terahertz wave utilizing the above-mentioned characteristics, there are spectral analysis technique for the interior of a substance, a transillumination imaging device as a safe substitute for an X-ray device, a nondestructive tomographic image acquisition technique for a lamellar structure, and the like.
Here, a source of wave for generating a terahertz wave (a terahertz wave generation apparatus that is constituted to include a light source such as a femto-second laser and a generation device such as a photoconductive antenna) can be classified into two types.
One of the types is a source of wave for generating a terahertz wave having a single color (single wavelength), including one generating a continuous wave (CW) and others generating a pulse wave. Here, the continuous wave is generated by combining two light beams having different frequencies so as to generate beating, which is supplied to the photoconductive antenna.
The other type is a source of wave for generating a monocycle terahertz pulse (having a single peak) containing wide band frequency components in which the light beams having different wavelengths are superimposed with each other keeping the phase relationship (in the state of the matched peak position).
Conventionally, as a device for generating a terahertz pulse, a photoconductive antenna including a pair of antenna type electrodes formed in a photoconductive semiconductor is suitably used.
First, femto-second laser pulse light is irradiated between electrodes to which a voltage is applied, and hence carriers are generated in a semiconductor. Next, the generated carriers are accelerated in the direction of the electric field (direction of the applied voltage). Then, a terahertz pulse having intensity corresponding to the acceleration of the carriers is generated and is radiated to the free space.
Such a photoconductive device is constituted of a semiconductor having high speed and high withstand voltage characteristics, such as GaAs (LT-GaAs) that is grown on a GaAs substrate at a low temperature.
In addition, a terahertz pulse generation device having a different form from the photoconductive antenna is disclosed in Applied Physics Letters, vol. 59, pp. 3357-3359, 1991. This device uses a silicon p-i-n diode.
Here, a film thickness of an intrinsic layer (an insulation layer or an i-layer that generates carriers when being irradiated with excitation light) of the device described above is adapted to be a thickness such that the excitation light can be sufficiently attenuated. Thus, almost all of the carriers excited by the excitation light can contribute to generation of the terahertz wave, and hence relatively high efficiency of generating the terahertz wave can be obtained.
In addition, a voltage is applied to the surface irradiated with the excitation light in a direction perpendicular to the surface (in a direction of the film thickness). In this case, if the film thickness is thin, high electric field can be applied efficiently compared with the conventional photoconductive antenna in which the voltage is applied in a direction along the surface of the film.
The device disclosed in the above-mentioned Applied Physics Letters, vol. 59, pp. 3357-3359, 1991 is configured so that the excitation light enters the surface of the diode. On the other hand, Microwave Photonics, 2003. MWP 2003 Proceedings. International Topical Meeting on, pp. 179-182, 2003 discloses a terahertz wave generator in which the excitation light enters a multilayer film structure of an InAlAs/InGaAs/InGaAsP p-i-n diode from an end surface of the device in a direction parallel to the film.
The structure of this device is aimed to generate beating by photomixing (heterodyne detecting) of two light beams having different wavelengths so that a single color terahertz wave can be generated efficiently. When the excitation light propagates in a waveguide structure having the InGaAs/InGaAsP layer (intrinsic layer or i-layer) as a core (layer for propagating excitation light), carriers are generated.
As to such waveguide type diode, a length of the intrinsic layer that can absorb the excitation light (absorption length) and the film thickness of the intrinsic layer (i-layer) can be designed independently of each other. Here, the absorption length is a parameter that contributes to the efficiency of generating the terahertz wave. The absorption length in case of the above-mentioned waveguide structure becomes a length in the direction parallel to the film from the incident end of the excitation light. In addition, the film thickness of the intrinsic layer can change an operating speed of the device. By decreasing the film thickness, the speed may be increased.
In addition, the generated terahertz wave propagates inside the waveguide structure, and hence it is easy not only to radiate into a free space but also to be connected to a transmission line.
However, in the photoconductive antenna disclosed in the above-mentioned Applied Physics Letters, vol. 59, pp. 3357-3359, 1991, energy of the light reflected by the surface for receiving the excitation light (intrinsic layer or i-layer) does not contribute to the generation of the terahertz wave, and hence an energy loss occurs.
In addition, if the terahertz wave having a higher power should be generated, it is required to increase the voltage to be applied and intensity (power) of the excitation light. In this case, a screening effect restricts the power of the generated terahertz wave.
Here, the screening effect means the following phenomenon that occurs when the photoconductive semiconductor is irradiated with the excitation light. That is the phenomenon that power of the generated terahertz wave is saturated as a power of the excitation light per unit area to be irradiated increases.
In addition, a case is considered in which the waveguide type diode disclosed in Microwave Photonics, 2003. MWP 2003 Proceedings. International Topical Meeting on, pp. 179-182, 2003 is applied to the device for generating the terahertz pulse. In this case, if the terahertz wave having a higher power should be generated, it is required to increase the voltage to be applied and the intensity (power) of the excitation light. The absorption layer (intrinsic layer or i-layer) and the core (layer for propagating excitation light) are formed as the same layer, and hence it is necessary to condense the excitation light when the excitation light enters the core. For this reason, the power of the generated terahertz wave is restricted by the above-mentioned screening effect.
In addition, the layer for generating the carriers (absorption layer) and the layer for propagating the excitation light (core) are formed as the same layer, and hence the propagation of the excitation light is restricted by transmittance of the absorption layer. Thus, the efficiency of propagation is restricted.